In the design of a low emission two stroke cycle engine, exhaust port timing control valves are used to control emissions and to provide desired low engine speed performance. A rotary valve disposed in the exhaust port between a cylinder bore and an outside of an engine block is the preferred valve configuration.
Typically the exhaust port is formed as part of the engine block casting. A pocket for the valve is machined in the exhaust port. A preferred method for machining the exhaust port valve pocket is to make a plunge cut from the outside of the block with a form cutter of the same general shape as the valve along an axis of the exhaust port. The exhaust valve is then passed into the exhaust port and rotatably mounted in the valve pocket.
This method of machining and valve installation requires that the exhaust port as cast be sized to accommodate the cutter and valve. The resultant exhaust port flow area between the valve and the outside of the block is larger than the flow area at the valve, and larger than the flow area of the exhaust manifold which mounts to the outside of the block. Exhaust flow passing through the exhaust port is subjected to an undesired volume expansion between the valve and the exhaust manifold. The expansion caused by the discontinuous increase in flow area causes the exhaust port to flow inefficiently.
Port designs which flow well, by contrast, have a near constant flow area along their length. Slight variation or divergence in the flow area along the length of the port will yield slightly better flow than a constant flow area. An exhaust port and valve according to this invention both enables the use rotary valves and plunge cutting and provides an efficient flow of exhaust therethrough.